Implement `templatefile()` in addition to the `file()` function within jobspecs

### Proposal

Implement Terraform's [`templatefile()`](https://developer.hashicorp.com/terraform/language/functions/templatefile) function for Nomad job specs, or equivalent with support for templating as part of [HCL spec](https://github.com/hashicorp/hcl/blob/main/hclsyntax/spec.md#template-interpolations).

### Use-cases

While Nomad has support for importing whole files as part of the `file()` function, it would be useful to be able to more directly combine it with for example [HCL2 input variables](https://developer.hashicorp.com/nomad/docs/job-specification/hcl2/variables) and/or client attribute. This would reduce the need for using an external file templating tool to generate the jobspec, or in the case of Terraform templating, stacking both `templatefile()` and Nomad variable interpolation on top of each other. I've encountered cases where I've had to write `$$${var}`, first to escape Terraform, then Nomad, then lastly render a file used for templating within the task.

### Attempted Solutions

It is possible to work around this limitation by passing variables to the [`meta`](https://developer.hashicorp.com/nomad/docs/job-specification/meta) block, and then accessing it via `{{ env "NOMAD_META_var" }}`, however it is a messy solution in comparsion to simply passing `${var}` to the file directly, especially if you want to do any more complex operations such as iterating over a list.

* Related issue: https://github.com/hashicorp/nomad/issues/7954

Start by reading the linked Terraform templatefile() documentation and the HCL template interpolation specification, then trace Nomad’s existing file() handling in jobspec evaluation. Compare the related issue #7954 and define how variables and client attributes should behave; done means a supported equivalent is implemented with coverage for the requested templating use cases.

Written by the indexing model from the issue text.